Earnings Per Share Attributable to Stockholders (Computation of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share Attributable to Stockholders [Abstract]
Basic, Net income attributable to stockholders	$ 23,138	$ 10,261	$ 85,993	$ 38,273
Effect of dilutive securities: Share units, Net income attributable to stockholders				(50)
Diluted, Net income attributable to stockholders	$ 23,138	$ 10,261	$ 85,993	$ 38,223
Basic, WA number of shares	108,365,427	107,120,618	107,811,391	106,993,358
Effect of dilutive securities: Share units, WA number of shares	551,313	214,304	408,006	337,570
Effect of dilutive securities: Stock options, WA number of shares	970,454	843,381	913,981	738,696
Diluted, WA number of shares	109,887,194	108,178,303	109,133,378	108,069,624
Basic, Per share amount	$ 0.21	$ 0.10	$ 0.80	$ 0.36
Effect of dilutive securities: Stock options, Per share amount		(0.01)	(0.01)	(0.01)
Diluted, Per share amount	$ 0.21	$ 0.09	$ 0.79	$ 0.35